The auditory system consists of ascending pathways that transmit information from the ear to the cortex, where sound is perceived, and descending pathways from the cortex that modulate the processing and flow of ascending information. The descending pathways are believed to play a role in a variety of functions, such as selective attention and discrimination of speech sounds, but understanding these roles is hindered by a lack of information on the neural circuitry. One of the largest descending pathways is from cortex to the inferior colliculus (IC), a prominent midbrain structure In at processes almost all ascending auditory information. Cortical projections activate subcortical circuits that use the neurotransmitters GABA and acetylcholine. These transmitters play prominent roles in the IC in normal function, in dysfunction such as tinnitus and in declining function associated with aging. The current proposal is focused on the relationships between descending cortical projections and the subcortical circuits that use GABA or acetylcholine to affect the IC. The proposed experiments will identify the source of acetylcholine inputs to the IC and characterize the cellular targets of those inputs. The experiments will also determine whether cortical proejctions make direct connections with the subcorical ceils that use GABA or acetylcholine and innervate the IC. The results will provide essential information on descending auditory pathways and specific subcortical circuits.